Broadband plane antenna with log-periodic reflectors

ABSTRACT

A broadband unidirectional linear or circularly polarized plane antenna with or without reflecting cavity is constructed with a plurality of log-periodically related reflecting rings or bands stacked behind the antenna elements symmetrically of the antenna axis. The conductive rings reflect backwardly directed radiation and comprise the exposed peripheral portions of a series of stacked plates, the dimensions and spacings of which increase progressively in increments of a predetermined ratio from a minimum adjacent the antenna elements to a maximum remote from the elements. In order to prevent the formation of an electromagnetic field in the spaces between axially aligned portions of the plates, field arresting material is located in these spaces.

Barbano n 3,745,585 1 July 10, 1973 BROADBAND PLANE ANTENNA WITH LOG-PERIODIC REFLECTORS [75] Inventor: Normand Barbano/Sunnyvale, Calif.

[73] Assignee: GTE Sylvania Incorporated,

Mountain View, Calif.

Primary Examiner-Eli Lieberman Attorney-Norman J. OMalley and John F. Lawler s71 ABSTRACT A broadband unidirectional linear or circularly polarized plane antenna with or without reflecting cavity is constructed with a plurality of log-periodically related reflecting rings or bands stacked behind theantenna elements symmetrically of the antenna axis. The conductive rings reflect backwardly directed radiation and comprise the exposed peripheral portions of a series of stacked plates, the dimensions and spacings of which increase progressively in increments of a predetermined ratio from a minimum adjacent the antenna elements to a maximum remote from the elements. In order to prevent the formation of an electromagnetic field in the spaces between axially aligned portions of the plates, field arresting material is located in these spaces.

13 Claims, 11 Drawing Figures PAIENTEUauuomn 3. 745.585

' sum 1 nr 4 FE-Z l BROADBAND PLANE ANTENNA WITH LOG-PERIODIC REFLECTORS BACKGROUND OF THE INVENTION This invention relates to antennas and more particularly to broadband unidirectional plane antennas.

Plane antennas such as the Archimedes spiral, equiangular spiral or flat log periodic types have the inherent advantage of being operable over a substantially broader bandwidth than is achievable with a dipole. A common practice for converting such a plane antenna from a bidirectional to a unidirectional radiator is to locate behind the antenna a cup which defines a resonant cavity. In order to extend operation of a cavitybacked antenna over a wider band of frequencies, the bottom of the cavity is often stepped in an attempt to make the height of the cavity behind the active regions of the antenna substantially equal to a quarterwavelength at the operating frequency. The steps are intended to cause the waves reflected from the cavity to have the same phase as the waves radiated in the same direction by the elements so that reflected and initially radiated waves combine constructively. An antenns of this general type is described in US. Pat. No. 3,555,554.

While the use of the stepped cavity has generally improved the performance of the antenna, it has also resulted in a periodic degradation of gain across the operating bandwidth of tha antenna. These clips in gain occur at the transition between cavity steps as the antenna elements aligned with these steps are energized across the band. Such nonuniformity in operation imposes'an undesirable limit on the effectiveness of the antenna.

Another problem encountered with cavity-backed plane antennas as discussed in the aforementioned patent is the generation within the cavity of radiation modes having higher frequencies than the fundamental mode of the antenna. These higher order modes cause deterioration of the radiation pattern with a consequent decrease in antenna gain in the vicinity of cavity resonance. Prior art attempts to reduce-or elimiate such higher order modes have generally been to dissipate their energy in wave absorbing material in the cavity. This practice inevitably reduces the overall gain of the antenna and accordingly such losses have long been recognized as a necessary. trade-off for uniform antenna operation over the band.

SUMMARY OF THE INVENTION An object of this invention is the provision of a broadband unidirectional plane antenna which has a relatively constant gain across the operating band.

Another object is the provision of a linearly polarized unidirectional plane antenna having a very broad operating bandwidth. v

A further object is the provision of.a plane unidirectional broadband antenna without a conventional cup reflector thereby reducing the overall weigt of the assembly.

In accordance with this invention, a plane antenna with or without a cavity is backed by log-periodically dimensioned reflectors to achieve broadband operation without degradation of gain.

DESCRIPTION OF DRAWINGS FIG. 1 is a top plan view of an equiangular spiral antenna embodying the invention;

FIG. 2 is a transverse section taken on line 2-2 of FIG. 1;

FIG. 3 is a top plan view of a linearly polarized antenna embodying the invention;

FIG. 4 is an enlarged perspective exploded view of the center feed connection for the antenna of FIG. 3;

FIG. 5 is an elevation partly in section of the antenna of FIG. 3;

FIG. 6 is a perspectivie view of an alternate form of reflector useful in the practice of the invention;

FIG. 7 is an enlarged transverse section taken on line 7-7 of FIG. 6;

FIG. 8 is a schematic perspective view of an antenna embodying the invention in which the cup reflector has been omitted;

FIG. 9 is a representation of gain characteristics of antennas embodying the invention;

FIG. 10 is a schematic top plan view of an Archimedes spiral antenna embodying the invention; and

FIG. 11 is a schematic side elevation of the antenna of FIG. 10.

' DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings and particularly to FIGS. 1 and 2, a preferred embodiment of the invention comprises a unidirectional plane antenna 10'hav ing two well known plane equiangular spiral conductive patterns 11 and12 formed on a dielectric sheetl3 and disposed in plane P over the open end of a cylindrical cup 14 having side wall 14a and a bottom wall 14b parallel to plane P. The antenna beam has an'axis normal to plane P and coincident with an extension of the cavity axis A. The spirals 11 and 12, preferably 0.002 inch copper film, are fed by lines 16 and 17 consisting of the inner conductorsof a pair of coaxial cables 18 and 19 which extend through a hollow post 20 coaxially of cup 14; lines 16 and 17 are directly electrically connected to the inner ends of the spirals, and the outer conductors of the cables are electrically floating. Balanced feed lines 16 and 17 are connected through a balun 22 to an unbalanced feed line 23 such as a coaxial line which in turn is connected to utilization apparatus, not shown. When energized by feed lines 16 and 17, theconductive spirals 11 and 12 radiate electromagnetic waves along axis A away from the cup and with circular polarization. The inner surface of cup wall 14a is covered with a radio frequency absorbing material 24 which prevents currents induced in this wall at the lower frequency limit of the operating'band from adversely affecting antenna performance.

In accordance with this invention, a plurality of plates 25 are supported on post 20 at different depths in and coaxially of the cavity in planes parallel to the plane P of the antenna spirals l1 and 12. In the embodiment shown in FIGS. 1 and 2, bottom wall 14b is the last plate of the group. The diameters of and spacings between adjacent plates 25 increase with distance from antenna spirals 11 and12 in accordance with a logarithmically periodic ratio 1' and are related by the constant scale factors a 2 tan 1 (d,,/X,,)

and plate spacing to plate diameter ratio ,./d,.)= (1 -1 2 tan 11/2 where r is a constant having a value less than one, a is the angle of convergence of lines through the outer edges of the plates, d, is the diameter of the adjacent larger plate, X,, the axial distance from point C of plate 11,, X is the corresponding distance to the plate d and 8,, is the spacing between plate d,, and the adjacent smaller plate. In practice, 1 has a value between an upper limit less than unity and a lower limit of approximately 0.7 and is selected empirically to provide a continuous field transfer from plate to adjacent plate as the antenna operating frequency is swept over the band. Stated differently, 1' is selected on the basis of providing a substantially constant gain characteristic, i.e., variations or dips in gain do not exceed a prescribed design limit such as 2 db.

Plates 25 are made of conductive material such as brass and are secured to the post by suitable means such as brazing. An important aspect of the invention is the provision of means described below to insure that only the outer peripheral ring portion of each plate functions as a reflector of electromagnetic waves from the corresponding antenna elements, such ring portion being defined as the upper plate surface (as viewed) having an inner diameter equal to the outer diameter of the next adjacent forward (smaller) plate and having axial line of sight exposure to the antenna elements. In other words, the overlapping portions of adjacent plates are rendered electrically inert. Thus only rings a of the lower five plates and of bottom wall 14b as shown in FIG. 3 are electrically effective to reflect and reinforce waves propagated by corresponding parts of the antenna sections.

In order to prevent the formation of secondary electric fields between adjacent plates, the space between plates is filled with a field arresting material 27 such as carbon impregnated polystyrene foam, commercially available under the trademark Eccosorb from Emerson and Cummings, lnc., Gardena, California. In the absence of such arresting material, electric currents induced in rings 25a by the electric fields produced by the antenna elements would tend to establish secondary electromagnetic fields in the spaces defined by the axially overlapping parts of adjacent plates. Such secondary fields between the plates would in turn produce electromagnetic waves which would tend to emanate radially andotherwise within the cavity so as to interfere with axially propagating waves. The field arrestor 27 inhibits the build-up of these fields and essentially isolates and renders electrically inert the space between plates. The flow of current is confined on plates 25 to rings 25a, thereby insuring that only these parts of the plates are electrically functional as reflectors relative to antenna sections 11 and 12.

It is important to note that the field arresting material 27 does not absorb energy and therefore its presence between the plates does not cause a loss of antenna power. Actual tests have verified that energy loss due to the presence of this material between the plates is negligible. Accordingly the gain of the antenna across the band is not adversely affected.

Another embodiment of the invention is shown in FIGS. 3,4 and 5 and comprises an antenna 29 having diametrically opposed log periodic sections 30 and 31 in a common plane P and disposed over the open end of cylindrical cup 14 as described above with antennas 10 except without the dielectric sheet. Sections 30 and 31 preferably are formed from 0.02 inch brass plate so as to be self-supporting and have their outer edges secured to the cup sidewall 14a by screws 32. A liner 33 of r.f. absorbing material is disposed on the cup side wall for the reason mentioned above.

Sections 30 and 31 comprise antenna elements 30a to 30f, inclusive, and 31a to 31f, inclusive, formed so that the size (length and width) and spacing of adjacent elements on each section decrease from a maximum value at the perimeter to a minimum near the center of the antenna in increments of a predetermined logarithmic ratio r so that lines through the outer ends of the elements intersect at the center point C. The log periodic antenna is noted for its psuedo frequency independent operation over a broad band of frequencies and is well known in the art. Corresponding elements of sections 30 and 31, for example, elements30a and 31a, lie on opposite sides of the diametral axis D of the sections and function electrically as dipoles having resonant frequencies corresponding to element dimensions.

The innermost ends 34 and 35 of sections 30 and 31, respectively, are spaced apart and are electrically connected to balanced feed lines 36 and 37, see FIG. 4, which preferably consist of the center conductors of coaxial cables and extend coaxially through cup 14; electrical connections between these center conductors and the inner ends 34 and 35 of the antenna sections are made through an insulator terminal 38 as shown in FIG. 4. A hollow post 39 extending coaxially through the cup cavity houses feed lines 36 and 37.

Mounted coaxially on post 39 within the cavity are a plurality of log periodically related reflecting plates 40 spaced apart by field arresting material 41. These plates and arresting material are constructed essentially the same as described above in conjunction with FIGS. 1 and 2 and function in the same manner so that outer rings 40a reflect electromagnetic waves emanating from antenna sections 30 and 31 along the antenna axis over the band without substantial variations in gain. The factor 1' for the log periodically related reflecting plates and for antenna sections 30 and 31 may be but are not necessarily equal, for former being derived empirically as mentioned above to hold gain variations or dips within prescribed limits over the operating frequency of the antenna.

An alternate form of reflector plate useful in the practice of the invention is shown at 43 in FIGS. 6 and 7. Such plate consists of a dielectric sheet of disk 44 having acentral feed post aperture 45 and a ring 46 of conductive material formed on the upper side (as viewed) preferably as a thin metallic film. The portion of that side of the disk between the aperture 45 and the conductive ring 46 is covered or coated with a field arresting material 47 such as graphite or the like. When a stack of log periodically related disks 43 are substituted for-plates 25 and 40 in FIGS. 2 and 3, respectively, with the rings 46 facing the antenna elements, it is not necessary to completely fill the space betwen plates with the field arresting material. The overall effeet of this modified plate construction is to provide the same electrical function as the previously described plates 25 and 40 while enabling more simplified manufacturing and assembly techniques.

In certain applications wherein radio frequency interference (RFI) is not a critical factor in the use of the antenna, cup 14 may be eliminated from the antenna assembly embodying the invention. A representation of such an embodiment is the antenna 48 shown in FIG. 8 in which the plane antenna sections 49 and 50, center fed by lines 51 and 52, are disposed over a stack of log periodically related coaxial plates 53 having field arrestingmaterial 54 therebetween. The plates 53 and arresting material 54 are constructed as described above. An example of an application of antenna 48 is its use as a feed for a parabolic reflector; the absence of the cup 14 does not interfere with the performance of the antenna system and, advantageously, reduces the weight of the assembly. Such weight saving is substantial at frequencies below 1.75 Gl-lz.

The absence of the cup from antenna 48 has also been found to result in a more constant gain characteristic over the operating band. Such characteristics are shown graphically in FIG. 9' in which curve 56 represents the relative gain of the antenna without the cup and curve 57 represents the gain of antenna 29 with a cup. It will be noted that curve 57 indicates that a higher'gain is obtained with the cup at the higher operating frequencies but that a drop in gain occurs at the lower limit of the band due to loss in the r.f. absorption liner in the cup. 7

Another embodiment of the invention is illustrated schematically in FIGS. and 11 by antenna 60 comprising a pair of plane center-fed radiators 61 and 62 in the form of Archimedes spirals, balanced feed lines 63 and 64 connected to the radiators and a reflector assembly 65 behind the radiators. Assembly 65 comprises stacked reflector plates or disks 66 that are related in diameter by a constant and that are separated by field arrestors 67 as described above. Spacings between plates 66 are constant as shown in FIG. 11. If desired, the antenna may be used with a back-up cup as indicated in the broken lines at 69.

By way of example, an antenna embodying the invention and constructed as shown in FIGS. 10 and 11 except with a cup and with log periodic spacing between plates had the following dimensions and physical characteristics:

Antenna radiators Material 0.005" copper on 0.03" fiberglass sheet Number of elements This antenna operated over a frequency band greater than 36 to 1, had a voltage standing wave ratio of less than 2 to l with respect to 50 ohms, and had an absotors, e.g., the limit is dictated at the lower end of the band by the maximum dimension that is practicable and at the high frequency end by the minimum practicable width of each element.

In addition to a large front-to-back ratio (i.e., high gain) and a low voltage standing wave ratio, antennas embodying the invention also exhibit a substantially constant beamwidth over the operating band.

What is claimed is:

1. In a plane unidirectional antenna having centerfed radiators in a common plane and a beam axis normal to said plane, the improvement of a plurality of stacked axially spaced reflectors supported transversely of an extension of said beam axis adjacent one side of said radiators, said reflectors being parallel to said plane and having transverse dimensions which increase in increments of a predetermined ratio with distance from said radiators, the spaces between adjacent reflectors containing electromagnetic field inhibiting material.

2. The antenna according to claim 1 with a dielectric disk for each reflector, each of said reflectors being formed as a conductive ring on the side of said disk facing said radiators, said material being applied as a layer to the portion of said side of the disk within said ring.

3. A plane unidirectional antenna comprising I a plurality of radiators in a common plane symmetrically disposed about an axis normal to said plane, means for coupling electromagnetic wave signals to and from said radiators, a plurality of coaxially disposed axially spaced plate adjacent to one side of said radiators, the transverse dimensions of said plates increasing progressively in increments of a predetermined ratio with an increase in distance from said radiators, and

electric field arresting material between overlapping portions of said plates.

4. The antenna according to claim 3 in which said radiators comprise linearly polarized log periodic sections, the spacing between adjacent plates increasing log periodically with distance from said radiators.

5. The antenna according to claim 3 in which each of said radiators is configured in the shape of an equiangular spiral, the spacing between adjacent plates increasing log periodically with distance from said radiators.

6. A plane unidirectional antenna comprising a pair of plane radiators symmetrically disposed about an axis normal to the plane of the radiators,

a pair of balanced feed lines connected to said radiators proximate to said axis,

a coaxial post containing said feed lines, and

a plurality of circular reflector elements supported on said post and axially aligned with said radiators whereby to produce a unidirectional radiation pattern,

said elements being axially spaced along said post and having dimensions parallel to said radiators increasing in increments of a predetermined ratio with axial distance from the radiators.

7. The antenna according to claim 6 with a conductive cup enclosing said reflector elements.

8. The antenna according to claim 6 in which said reflector elements comprise conductive rings, the inner diameter of the larger of a pair of adjacent rings being substantially the same as the outer diameter of the smaller ring of said pair.

9. In a plane unidirectional antenna having centerfed radiators in a common plane and a beam axis normal to said plane, the improvement of a plurality of stacked circular reflectors supported transversely of an extension of said beam axis adjacent one side of radiators, said reflectors being axially spaced from each other and parallel to said plane and having transverse dimensions and interreflector spacings which increase in increments of a predetermined ratio with distance from said radiators.

10. A plane unidirectional antenna comprising a pair of plane radiators symmetrically disposed about an axis normal to the plane of the radiators, a pair of balanced feed lines connected to said radiators proximate to said axis, j

a coaxial post containing said feed lines, and v a plurality of reflector elements supported on said postand axially aligned with said radiators whereby to produce a unidirectional radiation pattern, said elements being axially spaced along said post and having dimensions parallel to said radiators increasing log periodically with axial distance from the radiators, each of certain of said reflector elements comprising a peripheral conductive ring on a plate, the space between axially overlapping portions of adjacent plates containing electromagnetic field inhibiting material. 11. The antenna according to claim 10 in which said plates are formed of a dielectric substance and each-of 13. The antenna according to claim 12 in whichl each of said reflectors is configured in the shape of an Archimedes spiral. 

1. In a plane unidirectional antenna having centerfed radiators in a common plane and a beam axis normal to said plane, the improvement of a plurality of stacked axially spaced reflectors supported transversely of an extension of said beam axis adjacent one side of said radiators, said reflectors being parallel to said plane and having transverse dimensions which increase in increments of a predetermined ratio with distance from said radiators, the spaces between adjacent reflectors containing electromagnetic field inhibiting material.
 2. The antenna according to claim 1 with a dielectric disk for each reflector, each of said reflectors being formed as a conductive ring on the side of said disk facing said radiators, said material being applied as a layer to the portion of said side of the disk within said ring.
 3. A plane unidirectional antenna comprising a plurality of radiators in a common plane symmetrically disposed about an axis normal to said plane, means for coupling electromagnetic wave signalS to and from said radiators, a plurality of coaxially disposed axially spaced plates adjacent to one side of said radiators, the transverse dimensions of said plates increasing progressively in increments of a predetermined ratio with an increase in distance from said radiators, and electric field arresting material between overlapping portions of said plates.
 4. The antenna according to claim 3 in which said radiators comprise linearly polarized log periodic sections, the spacing between adjacent plates increasing log periodically with distance from said radiators.
 5. The antenna according to claim 3 in which each of said radiators is configured in the shape of an equiangular spiral, the spacing between adjacent plates increasing log periodically with distance from said radiators.
 6. A plane unidirectional antenna comprising a pair of plane radiators symmetrically disposed about an axis normal to the plane of the radiators, a pair of balanced feed lines connected to said radiators proximate to said axis, a coaxial post containing said feed lines, and a plurality of circular reflector elements supported on said post and axially aligned with said radiators whereby to produce a unidirectional radiation pattern, said elements being axially spaced along said post and having dimensions parallel to said radiators increasing in increments of a predetermined ratio with axial distance from the radiators.
 7. The antenna according to claim 6 with a conductive cup enclosing said reflector elements.
 8. The antenna according to claim 6 in which said reflector elements comprise conductive rings, the inner diameter of the larger of a pair of adjacent rings being substantially the same as the outer diameter of the smaller ring of said pair.
 9. In a plane unidirectional antenna having center-fed radiators in a common plane and a beam axis normal to said plane, the improvement of a plurality of stacked circular reflectors supported transversely of an extension of said beam axis adjacent one side of radiators, said reflectors being axially spaced from each other and parallel to said plane and having transverse dimensions and interreflector spacings which increase in increments of a predetermined ratio with distance from said radiators.
 10. A plane unidirectional antenna comprising a pair of plane radiators symmetrically disposed about an axis normal to the plane of the radiators, a pair of balanced feed lines connected to said radiators proximate to said axis, a coaxial post containing said feed lines, and a plurality of reflector elements supported on said post and axially aligned with said radiators whereby to produce a unidirectional radiation pattern, said elements being axially spaced along said post and having dimensions parallel to said radiators increasing log periodically with axial distance from the radiators, each of certain of said reflector elements comprising a peripheral conductive ring on a plate, the space between axially overlapping portions of adjacent plates containing electromagnetic field inhibiting material.
 11. The antenna according to claim 10 in which said plates are formed of a dielectric substance and each of said rings comprises a conductive layer on the surface of one of said plates facing said radiators, said field inhibiting material being disposed as a layer on said plate within said ring.
 12. In a plane unidirectional antenna having center-fed radiators in a common plane and a beam axis normal to said plane, the improvement of a plurality of stacked axially spaced reflectors supported transversely of an extension of said beam axis adjacent one side of said radiators, the spaces between adjacent reflectors containing an electromagnetic field inhibiting material.
 13. The antenna according to claim 12 in which each of said radiators is configured in the shape of an Archimedes spiral. 